docs: def from_json(cls, json_obj, variables=None):
          """
          Constructs a Variable from the provided json-object.
  
          Example
          --------
          >>> import json
          >>> with open("path_to_file.json") as infile:
          >>>     constraint = Constraint.from_json(json.load(infile))
          """
          if variables is None:
              variables = {}
          expression = parse_expr(json_obj["expression"], variables)
          if json_obj["indicator_variable"] is None:
              indicator = None
          else:
              indicator = variables[json_obj["indicator_variable"]]
          return cls(
              expression,
              name=json_obj["name"],
              lb=json_obj["lb"],
              ub=json_obj["ub"],
              indicator_variable=indicator,
              active_when=json_obj["active_when"]
          )